The following guide will let you deploy an instance of Froide on Heroku together with Postmark for email sending/receiving and S3 for file storage and static file serving.
Heroku and Postmark are free with limited capacity, S3 will cost you a tiny bit of money. You can also use any other file storage/serving service that is supported by Django Storages.

You should change these URLs through the SECRET_URLS setting of Froide.

Setup the MX records for your domain and make the API call like described in this article. The “Token” mentioned is your API key that you can find on the postmark page under Credentials.

For setting up sender signatures you should allow some hours to pass so that MX records are recognized. In Postmark click on “Sender Signatures” and add a new signature for your domain. You should setup two signatures: one for regular notification mails (e.g. mail@example.com, incoming mails going to a regular inbox of yours) and the other for sending out requests (request@inbound.example.com, incoming mails going to Postmark). You should also setup the DKIM and SPF records for your domain and verify them.

Since the confirmation mail for sending signatures of your request address will go to Postmark, check the “Inbound” tab of your “Activity” page of your Postmark account and use the confirmation link in the incoming email. This can take some time because of DNS propagation. You may have to resend the confirmation emails.